High ocean temperatures have led to cooling of US

The cooling of much of the continental US, the opposite effect of that experienced by much of the rest of the globe, has been due to higher ocean temperatures, say scientists.

A team of researchers, including scientists from the University of Illinois and the NASA Goddard Institute, used an atmospheric general circulation model to study the effects of changes in sea-surface temperature between 1950 and 1997 on regional cooling in the United States.

Though conventional wisdom has associated the lack of continent’s warming with the influence of man-made pollutants such as sulphate particles which have a cooling effect, the researchers found that by holding the levels of aerosols, solar irradiance and greenhouse gases constant, they could be eliminated as causes of the temperature decrease. The model only exhibited cooling when variations in tropical ocean temperatures were imposed, with the cooler regions becoming cloudier, indicating that the effect occurred due to moisture transport.

“Although portions of the US have failed to get warmer, they have gotten cloudier,” said Walter Robinson, a professor of atmospheric sciences at the University of Illanoi, and one of the researchers. “Our models suggest there is a strong correlation between this increased cloudiness and higher sea-surface temperatures in the tropical Pacific Ocean.”

This higher temperature releases additional water vapour, which is then swept across Central America and the Gulf of Mexico into North America, where it generates increased cloud cover over the central and eastern US, says Robinson. “By reflecting more solar radiation back into space, this additional cloud cover is directly responsible for the cooling,” he said.

The question remains, however, as to the cause of the heating of the ocean. “If the tropics are getting warmer because of global warming, then we can expect the cooling trend in the US to continue,” Robinson said. “But, if the effect is due to natural variability – and historical records indicate that recent temperature variations in the tropical Pacific are not unusual – then it’s only a matter of time before our temperatures ‘catch-up’ with the rest of the world.”

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